1. Field of the Invention
The present invention relates to sugical instruments for providing cryogenic temperatures.
2. Description of the Prior Art
Extremely cold temperatures may be used to destroy tissue for therapeutic purposes. This technique, commonly called "cryosurgery", has proven highly useful in the treatment of certain conditions that cannot be dealt with by the conventional surgical techniques of incision and excision. For example, small areas in the interior of the brain may be destroyed by cryosurgery in the treatment of Parkinson's disease. A needle-like probe is inserted in the brain to the desired location and filled with a cryogen to produce necrosis. The cryogen most often used is liquid nitrogen because of its low temperature (boiling point -196.degree. C, -321.degree. F), non-toxicity, and economy.
More recently, cryosurgery has been used to treat orthopedic conditions, for example, cancerous bone conditions or bone tumors which are many orders of magnitude larger than the brain areas noted above. An early orthopedic procedure involved exposing and removing the tumor by conventonal procedures and thereafter simply pouring liquid nitrogen directly into the cavity to kill the remaining abnormal cells.
While this procedure is resonably effective, it is difficult to control the size of the area which is frozen, to insure killing temperatures are reached, and to prevent injury to surrounding healthy tissues. A reproducible, controllable temperature curve cannot be obtained. To avoid these shortcomings, a conduit-like probe has been applied to the bone through which the liquid nitrogen is circulated to effect the freeze.
It will be appreciated that the conduit must be bent and shaped to closely conform to the tumor or bone area in order to achieve good heat transfer and freezing of the target area. In the past, copper has been used as the material for the probe because of its high thermal conductivity. Unfortunately, copper materials having the structrual strength necessary for cryosurgical applications are so stiff that tools are necessary to bend the probe. Forming the probe from tubing tended to work harden the material making it even more difficult to bend. The surgeon must consult x-rays to ascertain the size and shape of the tumor. In preparation for the surgery, the probe is hammered or bent with pliers into a configuration which conforms to the shape the tumor is believed to occupy.
Often, however, the size and shape of the tumor once exposed, differs from that indicated by the two dimensional x-rays so that the probe does not achieve a good heat transfer relationship with the tumor, making it difficult to rapidly and adequately carry out the operation. The shape of the probe cannot be easily altered because of the need for tools and the risk of rupture or collapse of the probe.
The poor heat transfer arises because of the presences of air pockets between the inappropriately formed probe and the tumor. Inasmuch as the thermal conductivity of air is 1/15,000th that of copper, the presence of even small air pockets is highly detrimental to heat transfer.
It is also necessary to closely control the feed rate of the liquid cryogen to obtain the desired temperature and to reduce operating time. In the past, a resistance heater has been employed in conjunction with the cryogen reservoir to boil a portion of the cryogen into the gaseous form to pressurize the flask and propel the cryogen through the probe. However, starting the cryogen flow produced in this manner is difficult because of the time lag between the energization of the resistance heater and creation of the gas pressure necessary to propel the cryogen. It is difficult to control, and particularly to vary, flow rates with such a technique. It may also needlessly prolong the operative procedure.